Iron deficiency is probably the most widespread nutritional problem in industrialized nations, yet there is a paucity of literature on the effects of iron deficiency on the ageing process. Because the population is continually exposed to chemicals in the environment and increasing quantities of therapeutic drugs are being consumed during the ageing process, it is imperative that studies be conducted to determine the interrelationship of diet and the ageing process on the drug-metabolizing system. We are approaching this problem by measuring the activities of specific enzymes associated with the mixed-function oxidase system in the microsomes of the liver and kidneys of rats. Four experimental groups are being used: (1) iron-replete males, (2) iron-deficient males; (3) iron-replete females, and (4) iron-deficient females. The enzyme activities being monitored are (1) cytochrome P-450, (2) heme oxygenase, (3) p-chloro N-methylaniline N-demethylase, (4) aniline hydroxylase, (5) benzopyrene hydroxylase, and (6) microsomal NADPH cytochrome C reductase. In addition, the effects of stannous chloride induction of heme oxygenase synthesis on the enzymes are being assayed. Heme oxygenase activity was stimulated threefold to fourfold in the liver and fourfold to fivefold in the kidney 24 hours after the injection of stannous chloride into six-month-old rats. The other enzyme activities were reduced twofold to fourfold 24 hours after the injection. The iron status of the animals is being determined by hemoglobin and hematocrit values and liver iron concentrations. At six months of age these values were lower in the iron-deficient animals compared to the iron-replete animals, as expected. In addition, body weight and food and water consumption data are being compiled as a function of age, sex and diet.